Computer systems generally utilize auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive read sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an “MR element”) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization of the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material separated by a layer of non-magnetic electrically conductive material are generally referred to as spin valve (SV) sensors manifesting the GMR effect (SV effect). In a spin valve sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO, FeMn, PtMn) layer. The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). In spin valve sensors, the spin valve effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium causes a change in the direction of magnetization in the free layer, which in turn causes a change in resistance of the spin valve sensor and a corresponding change in the sensed current or voltage. It should be noted that the AMR effect is also present in the spin valve sensor free layer.
FIG. 1 shows a typical spin valve sensor 100 (not drawn to scale) comprising end regions 104 and 106 separated by a central region 102. The central region 102 has defined edges and the end regions are contiguous with and abut the edges of the central region. A free layer (free ferromagnetic layer) 110 is separated from a pinned layer (pinned ferromagnetic layer) 120 by a non-magnetic, electrically-conducting spacer 115. The magnetization of the pinned layer 120 is fixed through exchange coupling with an antiferromagnetic (AFM) 125. An underlayer 126 is positioned below the AFM layer 125.
The underlayer 126, or seed layer, is any layer deposited to modify the crystallographic texture or grain size of the subsequent layers, and may not be needed depending on the substrate. A variety of oxide and/or metal materials have been employed to construct underlayer 126 for improving the properties of spin valve sensors. Often, the underlayer 126 may be formed of tantalum (Ta), zirconium (Zr), hafnium (Hf), or yttrium (Y). Ideally, such layer comprises NiFeCr in order to further improve operational characteristics. In particular, NiFeCr underlayer has been very successful in improving operational characteristics such as ΔR/R.
Free layer 110, spacer 115, pinned layer 120, the AFM layer 125, and the underlayer 126 are all formed in the central region 102. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed over hard bias layers 130 and 135, respectively, provide electrical connections for the flow of the sensing current Is from a current source 160 to the MR sensor 100. Sensor 170 is connected to leads 140 and 145 senses the change in the resistance due to changes induced in the free layer 110 by the external magnetic field (e.g., field generated by a data bit stored on a disk). IBM's U.S. Pat. No. 5,206,590 granted to Dieny et al. and incorporated herein by reference, discloses an MR sensor operating on the basis of the spin valve effect.
Another type of spin valve sensor is an anti-parallel (AP)-pinned spin valve sensor. FIG. 2A shows an exemplary AP-Pinned spin valve sensor 200 (not drawn to scale). Spin valve sensor 200 has end regions 202 and 204 separated from each other by a central region 206. AP-pinned spin valve sensor 200 comprises a Ni—Fe free layer 225 separated from a laminated AP-pinned layer 210 by a copper spacer layer 220. The magnetization of the laminated AP-pinned layer 210 is fixed by an AFM layer 208, or pinning layer, which is made of NiO. Again, beneath the AFM layer 208 is an underlayer 209. The laminated AP-pinned layer 210 includes a first ferromagnetic layer 212 (PF1) of cobalt and a second ferromagnetic layer 216 (PF2) of cobalt separated from each other by a ruthenium (Ru) anti-parallel coupling layer 214. The AMF layer 208, AP-pinned layer 210, copper spacer 220, free layer 225 and a cap layer 230 are all formed sequentially in the central region 206. Hard bias layers 235 and 240, formed in end regions 202 and 204, provide longitudinal biasing for the free layer 225. Electrical leads 245 and 250 are also formed in end regions 202 and 204, respectively, to provide electrical current from a current source (not shown) to the spin valve sensor 200.
Various parameters of a spin valve sensor may be used to evaluate the performance thereof. A couple of examples of such parameters are the structure sheet resistance (R) and GMR ratio (ΔR/R). The GMR ratio (ΔR/R) is defined as (RAP−RP)/RP, where RAP is the anti-parallel resistance and RP is the parallel resistance.
Numerous theoretical studies have attempted to explain the behavior of spin valve effects. However, there does not currently exist an explanation of the main factors controlling the magnitude of the sensor response, as characterized by ΔR/R, as it relates to the required properties of the conductive spacers and ferromagnetic (FM) layers constituting such device. Experimental efforts have been largely based on trial and error, by investigating various combinations of FM layers and conductive spacer layers. None of the previous work has yielded quantitative guidelines for the maximization of ΔR/R for spin valve sensors by providing selection criteria for the layer compositions of the FM material and the conductive spacer.
What is known is that the GMR effect depends on the angle between the magnetizations of the free and pinned layers. More specifically, the GMR effect is proportional to the cosine of the angle β between the magnetization vector of the pinned layer (MP) and the magnetization vector of the free layer (MF) (Note FIGS. 2B and 2C). In a spin valve sensor, the electron scattering and therefore the resistance is maximum when the magnetizations of the pinned and free layers are antiparallel, i.e., majority of the electrons are scattered as they try to cross the boundary between the MR layers. On the other hand, electron scattering and therefore the resistance is minimum when the magnetizations of the pinned and free layers are parallel; i.e., majority of electrons are not scattered as they try to cross the boundary between the MR layers.
In other words, there is a net change in resistance of a spin valve sensor between parallel and antiparallel magnetization orientations of the pinned and free layers. The GMR effect, i.e., the net change in resistance, exhibited by a typical prior art spin valve sensor is about 6% to 8%.
However, over time, the properties of a read head change. Because the read head is in essence a resistor, heat is generated as current passes through the head, leading to thermal degradation. The current also produces a magnetic field, which can magnetize the free layer of the head, also changing the properties of the head.
FIG. 3 is a chart 300 showing illustrative stress test data for a nominally identical group of heads with currents for the individual heads chosen so that that the maximum internal temperature of each head is 225 degrees C., with the external ambient temperature at 120 degrees C.
Over the first few hours of use, the properties of the head change quickly, such that the normalized amplitude of the current through the read head rapidly changes. This phenomenon is known as “short term amplitude spreading,” and is indicated on FIG. 3 as occurring during time period 302. It would be desirable to accelerate the short term spreading so that head operation is more stable and more uniformly behaved from the first use.
The short-term amplitude spreading phenomenon also makes testing heads very expensive, as to perform an adequate test, current must be passed through the head for the short term spreading period. If the head is tested prior to short term decay, a bad head may test out as being good, or vice versa. Thus, the manufacturer must energize the heads for long periods of time prior to testing or run the risk of sending out a defective product. Thus it would be desirable to accelerate the short term decay so that the head may be tested in a state closer to how it will operate during actual use.
As shown in FIG. 3, the response of the amplitude in time (A/Ao is the amplitude divided by the initial amplitude) is different for each head. For short times, less than 10 hours, that average amplitude may increase or decrease in a fast rate, while for longer times there is a more-or-less uniform decay, at typically slower rate, of the amplitude.
Short Term Amplitude Decay.
Short term amplitude decay is at least partially due to a magnetic relaxation from the state in which the heads were manufactured to a stable state in use conditions. This relaxation is driven, in part, by the magnetic field from the current and is accelerated by temperature (from the combination of the Joule heating by the current and the ambient temperature). Essentially, the process of magnetic annealing is introduced when the heads are in use.
The effect of magnetic relaxation can be further realized from the polarity of the bias current. It is known that the direction of magnetic field from the bias current is current polarity sensitive. The effect is demonstrated in FIG. 6 (discussed in more detail below). In FIG. 6, the average short term behavior on a group of 10 to 15 heads is described using the average of normalized amplitude versus stress time. It is clearly seen that the normalized amplitude is either decrease or increase in a fast rate when the polarity is switched.
It would be desirable to remove or reduce amplitude spreading. The result would be a more uniform distribution of heads, which in turn would allow the reads to run at a higher bias voltage, producing more amplitude and reduced error rates.
After the quick short term decay, the current continues to decay during a “long term decay” period. Eventually, the current through the head stabilizes, but this can take a long time (several hundred hours).
What is needed is a way to push the head towards a stable condition before being used in a system. Also needed is a way to improve the deviation between sensors for how they behave in magnetic performance with time in files. What is also needed is a process that improves the magnetic performance of sensors by allowing more bias voltage to be applied through the sensors without trading off thermal stability.